An oxygen sensor, which detects the concentration of oxygen within exhaust gas, is known as a gas sensor used for improving the fuel consumption of an internal combustion engine of an automobile or the like and/or for performing combustion control for the engine. Further, there has been demanded reduction in the amount of nitrogen oxides (NOx) within exhaust gas in order to cope with the strengthened exhaust gas regulation for automobiles, and NOx sensors which can directly measure NOx concentration have been developed.
Such a gas sensor includes a gas sensor element having one or more cells each composed of oxygen-ion conductive solid electrolyte layer formed of zirconia or the like, and a pair of electrodes formed on the surface of the solid electrolyte layer. The gas sensor is configured to detect the concentration of a specific gas on the basis of the output from the gas sensor element.
One known example of such a gas sensor is a wide-range air-fuel-ratio sensor (hereinafter may be referred to as a “UEGO sensor”), which is configured such that two cells (an oxygen-concentration detection cell and an oxygen pump cell) are disposed with a measurement chamber disposed therebetween, and a gas to be measured (hereinafter referred to as an “object gas”) is introduced into the measurement chamber via a diffusion resistor so as to detect oxygen contained in the object gas. Further, another known example of such a gas sensor is an NOx gas sensor which includes three cells in total; i.e., the above-described two cells (an oxygen-concentration detection cell and an oxygen pump cell), and a cell for detecting NOx gas concentration.
A sensor drive circuit is connected to such a gas sensor so as to supply electricity to cells via the sensor drive circuit, and the concentration of a specific gas contained in the object gas is measured on the basis of the output of a cell. A control apparatus, including the sensor drive circuit, is called a “gas sensor control apparatus.” Further, there are various states in which electricity is supplied to cells (hereinafter referred to “energization states”), including an energization state for protection which is provided for protecting the gas sensor; a pre-activation energization state in which a minute current is supplied to the gas sensor in an non-activated state; and an energization state for gas concentration measurement which is provided for measurement of the concentration of a specific gas.
Of these energization states, the energization state for protection is established in an operation mode in which the electrical continuity between the cells and the sensor drive circuit is cut off so as to stop the flow of current to the gas sensor, to thereby protect the gas sensor. Further, the pre-activation energization state is established in an operation mode in which a minute current is supplied to the oxygen-concentration detection cell so as to accumulate oxygen of a reference concentration in the reference oxygen chamber of the oxygen-concentration detection cell, to thereby prepare for gas concentration measurement.
Incidentally, the sensor drive circuit or wiring lines (electricity supply lines) of the gas sensor may suffer a wiring anomaly, such as wire breakage or short-circuiting with a battery or the ground. When, irrespective of occurrence of such a wiring anomaly, the energization state for gas concentration measurement is continued so as to measure the concentration of the specific gas, an excessively large current flows through the gas sensor, and the gas sensor may be broken.
In view of the above, there has been developed a technique for cutting off the electrical connection between the sensor drive circuit and the gas sensor upon detection of a wiring anomaly so as to bring the gas sensor into an energization state for protection, and then diagnosing the wiring anomaly so as to determine the details of the anomaly and the location where the anomaly has occurred (see Patent Document 1). This technique prevents an over current from continuously flowing to the gas sensor even after a wiring anomaly has occurred, to thereby prevent breakage of the gas sensor.
Further, when an instruction for switching to the energization state for gas concentration measurement from another energization state is output to the sensor drive circuit without awareness of a wiring anomaly, similarly, an excessively large current flows to the gas sensor, and the gas sensor may be broken.
In view of the above, there has also been developed a technique for permitting the switching to the energization state for gas concentration measurement only when the state immediately before switching is the pre-activation energization state, to thereby allow detection of an anomaly without breaking the gas sensor (see Patent Document 2). If a wiring anomaly occurs in the pre-activation energization state in which a minute current is supplied to the gas sensor, the voltage applied to the gas sensor deviates from a normal range, whereby a wiring anomaly can be detected.